Add png extension only if file exist when load local image (#46971)

Summary:
FIxes #46870.

To avoid many breaking changes, I modified the logic to add the PNG extension only if the file exists.

## Changelog:

[IOS] [FIXED] - Add png extension only if file exist when load local image

Pull Request resolved: #46971

Test Plan: Demo in #46870 .

Reviewed By: blakef

Differential Revision: D65276409

Pulled By: cipolleschi

fbshipit-source-id: 9e03a36b229f7c336632b5b76a92d9f749c59835

packages/react-native/Libraries/Image/RCTImageLoader.mm

@@ -477,7 +477,10 @@ - (RCTImageURLLoaderRequest *)_loadImageOrDataWithURLRequest:(NSURLRequest *)req
 
     // Add missing png extension
     if (request.URL.fileURL && request.URL.pathExtension.length == 0) {
-      mutableRequest.URL = [request.URL URLByAppendingPathExtension:@"png"];
+      NSURL *pngRequestURL = [request.URL URLByAppendingPathExtension:@"png"];
+      if ([[NSFileManager defaultManager] fileExistsAtPath:pngRequestURL.path]) {
+        mutableRequest.URL = pngRequestURL;
+      }
     }
     if (_redirectDelegate != nil) {
       mutableRequest.URL = [_redirectDelegate redirectAssetsURL:mutableRequest.URL];